Photochromic imaging utilizing tetrazolium compounds

ABSTRACT

Imagewise actinic light exposure of a layer of a composition comprising a tetrazolium compound yields a negative image of an intensely colored formazan compound. The image thus formed may be readily fixed by contact with a composition comprising an oxidizing agent which yields water-insoluble reduction products of contrasting color.

United States Patent [191 Levinos PHOTOCI-IROMIC IMAGING UTILIZING TETRAZOLIUM COMPOUNDS [75] Inventor:

[73] Assignee: Keuffel & Esser Company,

Morristown, NJ.

[22] Filed: Dec. 3, 1973 [21] Appl. No.: 421,180

Steven Levinos, Summit, NJ.

[52] US. Cl. 96/48 R; 96/90 R [51] Int. Cl. G03C 5/24 [58] Field of Search 96/88, 90 R, 48 R, 48 QP [56] References Cited UNITED STATES PATENTS 3,278,366 10/1966 Schiele 96/l.5 3,655,382 4/l972 Brault et a]. 96/48 R FOREIGN PATENTS OR APPLICATIONS 908,299 10/1962 United Kingdom 96/48 [451 July 29,1975

670,883 4/1972 United Kingdom 96/48 OTHER PUBLICATIONS Zeitschrift fur Naturforschung, 3B, (1948), 379.

Primary Examiner-Won H. Louie, Jr. Attorney, Agent, or Firm-Lionel N. White, Esq.

[5 7 ABSTRACT 6 Claims, No Drawings PHOTOCI'IROMIC IMAGING UTILIZING TETRAZOLIUM COMPOUNDS BACKGROUND Numerous photochromic compounds have heretofore been known and employed in imaging systems. The chromotropic nature of these materials, however, often results in the formation of unstable images; that is to say, images derived from light exposure of photochromic materials, unless immediately fixed by involved procedures, revert spontaneously or by oxidation to a color or tone which is indistinguishable from the unexposed composition.

Thus the reversible reactions involving chromotropic materials such as noted in US. Pat. No. 2,772,158 and US. Pat. No. 2,761,846 are not entirely suited to the formation of intensely colored, permanent images in simple, economical processes.

SUMMARY The present invention provides materials which may be readily imaged by actinic light exposoure, and provides a simple means for fixing such images and thus eliminates a serious disadvantage of prior photochromic imaging methods.

The invention utilizes a tetrazolium compound to provide a coating composition which, upon imagewise exposure to actinic light, yields a colored image comprising an oxidizable chromotropic formazan compound. Subsequent to such a formation of formazan compound, the imaged layer is contacted with an oxidizing agent which readily yields distinctly colored reduction products, preferably of a permanent, insoluble nature. Such a fixing agent is an aqueous solution of a ferric ferricyanide complex which, upon contact with the oxidizable formazan image, provides a similar imagewise disposition of highly colored water-insoluble ferric ferrocyanide. A water rinse may be utilized during the processing of the imaged sheet to remove soluble unexposed tetrazolium composition and ensure a clean, stable background for the permanent, nonfading color image in the coated material.

DESCRIPTION A broad range of tetrazolium compounds useful in the present invention are commercially available as the chloride or bromide salts. Due to the availability of useful compatible coating binders and adjuncts of a hydrophilic nature, and in view of the economy of aqueous coating and processing vehicles, the more water soluble of these compounds are preferred. Of particular utility in this respect are the 2,3,5-triphenyl-ZI-I-tetrazolium chloride and 3,3-(4,4'-biphenylene)-bis-(2,5- diphenyl)-2H-tetrazolium chloride (or neotetrazolium chloride) compounds whose photochemically derived formazans are respectively red and purple.

Other tetrazolium salts which may be employed in the compositions of the present invention include 3,3 dianisole-bis-4,4'-(3,5-diphenyl) tetrazolium chloride; 2,5-diphenyl-3-( 1-naphthyl)-2I-I-tetrazolium chloride; 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl-2H- tetrazolium chloride; 3-(4,5-dimethylthiazolyl-2-)-2,5- diphenyl-ZH-tetrazolium bromide; and (2,2'-di-pnitrophenyl-5,5'-di-p-thiocarbamylphenyl-3,3)-(3,3- dimethoxy-4,4'-biphenylene) ditetrazolium chloride.

In the preparation of imaging material, notably coated sheet products which may be effectively processed in existing exposure and post-treatment equipment, common techniques may be employed. Thus, selected tetrazolium compounds are combined in coatable compositions, preferably in the form of solutions containing coating aids or binder materials such as gelatin, and the resulting composition is coated, with subsequent drying, upon any of a number of available base sheet supports used in the photographic industry. In order to improve the reducing environment conducive to the photochromic reaction, it is useful to employ supports of a cellulosic character such as paper, regenerated cellulose, and the like. This preferable condition can further be enhanced by the incorporation in the coating composition of electron donor compounds such as sodium formate, dextrose, glycerin, or glycols. Glycerin and glycol humectant components have been found to be particularly useful in the present compositions due to their additional ability to retain moisture within the coated composition and thereby provide yet another desirable property in the medium for the colorforming photochromic reaction.

A brief exposure of the coated material to a patterned image of actinic radiation, particularly light in the short wavelength, near ultraviolet range, results in the formation of highly colored, water-insoluble formazan compounds in the light-exposed areas. A water rinse of the material thus imaged effectively removes a substantial portion of the unexposed tetrazolium compound from the composition in preparation for the application of a color-forming oxidizing agent to effect a fixing of the image defined by the disposition of formazan compound in the coated layer. While such a prerinse is not necessary to the fixing of the formazan image, it is preferred in order to remove soluble tetrazolium compounds and thus limit adulteration of the subsequently applied fixing solution.

Fixing of the image to prevent color loss from oxidation and reversion to the less vivid tetrazolium form is then accomplished by the application of an oxidizing agent, preferably in aqueous solution, which, in the process of oxidizing the formazan image material, is itself reduced to an intensely colored insoluble form which replaces entirely, or depending upon the concentration of the oxidizer in the fixing solution, mixes with the colored formazan product to provide a permanent image of a preferred neutral tone. Particularly effective in the role of such a fixing agent is the water-soluble ferric ferricyanide complex which upon reduction yields the insoluble, cool toned Prussian blue color body. A means by which the color tone of the final fixed image may be varied when using such a fixing agent resides in the extent of the original light exposure, particularly with those tetrazolium compounds yielding red or similar warm-toned formazan products. Thus, less intensive light exposures result in the cooler, more neutral tone final images.

Other fixing materials may be readily derived from the existing art without extensive experimentation, bearing in mind the desired result of a substantially insoluble, stable, and intensely colored reduction product. Such an additional useful material is a dilute Fehlings solution which provides permanent images of a brown tone finding utility in intermediate original imaging.

The insoluble character of the fixed image material allows for a final water rinse which is preferred in most circumstances to thoroughly cleanse the imaged material of the various applied reactants as well as, in some circumstances, to remove unexposed coated material to yield useful relief images.

PREFERRED EMBODIMENTS The following examples will serve to further describe the manner of preparing the light-sensitive compositions and imaging materials of the present invention and to illustrate the manner in which such materials are employed in the formation of useful images.

EXAMPLE I A solution of the following composition was prepared:

Deionized water 50 ml. 2,3-5-triphenyl-2I-I-tetrazolium 2.5 g Dextrose (anhydrous) 0.5 g Sodium formate 0.5 g Glycerin 1.0 g

A sheet of porous paper was immersed in the solution and, after draining, was air dried in subdued light. The resulting material was exposed for 60 seconds through an opaque stencil to light from a source rich in ultraviolet radiation (a bank of GE. F8T5 BLB lamps about 12 mm apart) located about mm from the exposure plane. The sheet was then washed with water for about 20 seconds to reveal a red negative image of the stencil. The imaged sheet was thereafter immersed for about seconds in an aqueous solution of 5 percent by weight potassium ferricyanide and 5 percent by weight of ferric nitrate 9HzO. The sheet was then given a final water rinse and dried to yield a dark gray-brown image in a clean background.

EXAMPLE II A solution of the following composition was prepared:

Deionized water ml. Neotetrazolium chloride 1.25 g. Dextrose (anhydrous) 0.25 g. Sodium formate 0.25 g. Glycerin 0.50 g.

Paper was immersed in the resulting solution, drained, and dried in the previous manner. After exposure for about 2 minutes under conditions described in Example I, the sheet was rinsed with water for about 30 seconds and immersed in a ferric ferricyanide solution as previously described. The initial purple formazan image of notably lower density than that observed in Example I, this as a result of the lower concentration of tetrazolium salt in the instant photosensitive composition, was converted to a distinct permanent image of nearly neutral tone. A final water rinse and drying completed the processing.

EXAMPLE III A composition of Example I was modified to the extent of increasing the sodium formate and glycerin components to 1.0g. and 2.5g. respectively, and sheet material was prepared, exposed, and processed as there described. An image of substantially deeper density was obtained, thus indicating beneficial effects of increased reducing environment and moisture content upon the photosensitive compositions of the present invention.

EXAMPLE IV Imaging material was prepared in accordance with the procedure set forth in Example I and was exposed for about seconds to the light source arrangement there described. Immersion of the formazan-imaged sheet for about 60 seconds in a dilute Fehlings solution provided a dark brown image which, after a final rinse and drying, increased in a tonal density over a period of several hours.

EXAMPLE V A coating formulation of the following composition was prepared:

Deionized water 50 ml. Gelatin (inert, high bloom) 2.5 g. 2,3,5-triphenyl-2I-I-tetrazolium chloride 2.5 g. Sodium formate 0.5 g. Glycerin 1.0 g.

Each of the ingredients was dissolved in the water in the order given above, and the composition was coated to a wet thickness of about l30p.m on a pre-subbed polyethylene terphthalate film base support commonly employed in the preparation of photographic products. After the coating was air dryed in the absence of light, the material was exposed through the base sheet for 2.5 minutes in the manner described in Example I, and the unexposed nonimage portions of the coating were removed by washing with running water at a temperature of about 30C. The material was then immersed for about 90 seconds in the previously described ferric ferricyanide solution whereupon the initial bright red relief image was converted to one of gray-brown tone. The processing was completed by a short water rinse and air drying.

What is claimed is:

l. The method of forming a permanent image which comprises:

a. providing light-sensitive imaging material comprising a support and a coating thereon of a composition comprising a light-sensitive component consisting essentially of a water-soluble tetrazolium compound;

b. imagewise exposing said material to actinic light rich in ultraviolet radiation for a time sufficient to convert said tetrazolium compound in the lightexposed areas of said composition to a colored, water-insoluble formazan compound;

0. rinsing said material to remove water-soluble components from said composition; and

d. contacting at least the light-exposed areas of said material with a fixing agent comprising an oxidizer capable of yielding distinctly colored waterinsoluble reduction products, thereby at least partially oxidizing said formazan compound and reducing said oxidizer to form said colored reduction products in said light-exposed areas.

2. The method according to claim 1 wherein the coated surface of said support is a cellulosic substance selected from the group consisting of paper and regenerated cellulose.

3. The method according to claim 1 wherein said composition further comprises an electron donor material selected from the group consisting of sodium formate, glycerin, dextrose, and glycols.

4. The method according to claim 3 wherein said composition further comprises an hydrophilic colloid binder.

5. A method according to claim 1 wherein said fixing agent is selected from the group consisting of aqueous ferric ferricyanide solution and Fehlings solution.

6. A method according to claim 5 wherein said imaging material comprises a tetrazolium compound selected from the group consisting of 2,3,5-triphenyl-2H- 

1. THE METHOD OF FORMING A PERMANENT IMAGE WHICH COMPRISES: A. PROVIDING LIGHT-SENSITIVE IMAGING MATERIAL COMPRISING A SUPPORT AND A COATING THEREON OF A COMPOSITION COMPRISING A LIGHT-SENSITIVE COMPONENT CONSISTING ESSENTIALY OF A WATER-SOLUBLE TETRAZOLIUM COMPOUND, B. IMAGEWISE EXPOSING SAID MATERIAL TO ACTINIC LIGHT RICH IN ULTRAVIOLET RADIATION FOR A TIME SUFFICIENT TO CONVERT SAID TETRAZOLIUM COMPOUND IN THE LIGHT-EXPOSED AREAS OF SAID COMPOSITION TO A COLORED, WATER-INSOLUBLE FORMAZAN COMPOUND, C. RINSING SAID MATERIAL TO REMOVE WATER-SOLUBLE COMPONENTS FROM SAID COMPOSITION, AND D. CONTACTING AT LEAST THE LIGHT-EXPOSED AREAS OF SAID MATERIAL WITH A FIXING AGENT COMPRISING AN OXIDIZER CAPABLE OF YIELDING DISTINCTLY COLORED WATER-INSOLUBLE REDUCTION PRODUCTS, THEREBY AT LEAST PARTIALLY OXIDIZING SAID FORMAZAN COMPOUND AND REDUCING SAID OXIDIZER TO FORM SAID COLORED REDUCTION PRODUCTS IN SAID LIGHT-EXPOSED AREAS.
 2. The method according to claim 1 wherein the coated surface of said support is a cellulosic substance selected from the group consisting of paper and regenerated cellulose.
 3. The method according to claim 1 wherein said composition further comprises an electron donor material selected from the group consisting of sodium formate, glycerin, dextrose, and glycols.
 4. The method according to claim 3 wherein said composition further comprises an hydrophilic colloid binder.
 5. A method according to claim 1 wherein said fixing agent is selected from the group consisting of aqueous ferric ferricyanide solution and Fehling''s solution.
 6. A method according to claim 5 wherein said imaging material comprises a tetrazolium compound selected from the group consisting of 2,3,5-triphenyl-2H-tetrazolium chloride; 3,3-(4,4''-biphenylene)-bis-(2,5-diphenyl)-2H-tetrazolium chloride; 3,3''-dianisole-bis-4,4''-(3,5-diphenyl) tetrazolium chloride; 2,5-diphenyl-3-(1-naphthyl)-2H-tetrazolium chloride; 2-)p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl-2H-tetrazolium chloride; 3-(4,5-dimethylthiazolyl-2-)-2,5-diphenyl-2H-tetrazolium bromide; and (2,2''-di-p-nitrophenyl-5,5''-di-p-thiocarbamylphenyl-3,3''), 3, 3-dimethoxy-4''-biphenylene) ditetrazolium chloride. 